1. Field of the Invention
The present invention relates to a circuit pattern forming method and a circuit pattern forming device, to form circuit patterns on circuit boards used in electronic devices, electrical devices, computers and communication devices, and, more particularly, to a circuit pattern forming method and a circuit pattern forming device that form circuits by ejecting a circuit pattern forming solution onto a substrate.
2. Description of the Related Art
Printed circuit boards installed in electronic devices, communication devices and computers are mounted with a variety of kinds of semiconductor devices, such as LSIs, and of electronic components. Many kinds of printed circuit boards are available, including those using ceramics as a base material, those using as a base material a composite material formed of a reinforcement material, such as glass fibers and a synthetic resin, such as epoxy resin, and those using a flexible film, such as polyester resin and aramid resin, as the base material. Most of the conventional printed circuit boards have used a one-sided or double-sided substrate. As electronic devices have been miniaturized and packed in higher density, laminated circuit patterns have come to be used. At present eight- or sixteen-layer printed circuit boards are becoming a mainstream. At the same time, circuit patterns are also being formed of increasingly finer circuits, and their density is rapidly increasing.
Japanese Patent Application Laid-Open No. 07-245467 (1995) discloses a method of forming circuits by directly ejecting a conductive solution and an insulating solution onto a surface of a substrate by an ink jet method, to draw conductive patterns and insulating patterns. This method obviates a subtractive method, which, in turn, offers an advantage of being able to reduce the number of steps required and to eliminate the need to process a waste liquid produced during plating and etching steps.
In Japanese Patent Application Laid-Open No. 07-245467 (1995), referenced above, circuit patterns are formed by ejecting ink droplets from an ink jet printer, such that adjoining droplets on a substrate overlap each other. This is because, if the adjoining droplets fail to overlap, those portions that are out of contact with each other will lead to an open circuit. With this method, however, droplets ejected onto the substrate contact and merge with the existing droplets on the substrate and may form liquid puddles.
Let us explain the liquid puddles thus formed.
FIG. 18 shows liquid puddles 201, 203 formed on circuit patterns 200. The puddles 201, 203 are formed by the droplets ejected onto the substrate merging with and becoming attracted to the existing droplets that have already landed on the substrate. Further, as the liquid puddles expand, the puddles themselves contact other patterns, as indicated at 203, causing short-circuits. On the other hand, when the ejected droplets land at positions where they fail to contact the existing droplets on the substrate because of ejection errors of an ejection head, these droplets become fixed, isolated from the patterns that they are supposed to contact, which will cause an open circuit, as indicated at 202 of FIG. 18.
Under these circumstances, proposals have been made to prevent forming of such liquid puddles and, therefore, the problems of open-circuits and short-circuits, as disclosed in Japanese Patent Application Laid-Open No. 2003-133691. This method, as shown in FIGS. 19A and 19B, ejects droplets 1003 from the head 1002 to form dots 1006 on the substrate 1001 in a first ejection step. The dots 1006 are formed at a greater pitch 1005 than a diameter 1004 of the dots formed by the droplets landing on the substrate 1001. The pitch refers to a distance between the centers of the adjoining dots.
Then, the printed substrate is subjected to a drying step to fix the dots 1006 formed by the first ejection step. After this, a second ejection step is performed to form dots 1007. In forming the dots 1007, droplets 1003 are ejected to positions different from those of the first ejection step at the same pitch 1005 as the first ejection step. Then, the substrate is dried again to fix the dots 1007 formed in the second ejection step. After this, dots 1008 are formed in a third ejection step. In forming the dots 1008, droplets 1003 are ejected at a smaller pitch than the pitch 1005 of the first ejection step.
As described above, in Japanese Patent Application Laid-Open No. 2003-133691, dots are formed on the substrate at intervals and then dried, after which, droplets are ejected between the formed dots. With this arrangement, the landing droplets are not attracted to the existing dots on the substrate, preventing the formation of liquid puddles. Further, Japanese Patent Application Laid-Open No. 2003-133691, as shown in FIG. 19B, increases the thickness of circuits formed, to eliminate such problems as open-circuits and short-circuits.
The technology of Japanese Patent Application Laid-Open No. 2003-133691, however, has a problem that, since the drying step is executed after each ejection step, and the patterns are formed thick, the thickness of the circuit patterns after the third ejection step becomes uneven, impairing the flatness of the surface. This technology is, therefore, not suited for forming circuit patterns that are thin, uniform in thickness and flat, which are essential requirements in forming multilayered circuit boards. In recent years, circuit boards are formed with an increasingly higher level of circuit density, and there are growing needs for circuit patterns that can be formed of thin films in a multilayer structure. These demands, however, cannot be met with the technology of Japanese Patent Application Laid-Open No. 2003-133691.